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2002 Cavalier won't start. Loud bang.

01-24-2009, 08:34 PM

Hi All. I have a 2002 Cavalier that periodically has a very hard time starting. Once it starts, it takes a few seconds to stop "sputtering" but then runs great. The last time I tried, it wouldn't start at all. I took out the CNG fuse and it starts great with unleaded gasoline. Also, I will periodically hear an explosion sound under my hood while trying to start it. When that happens, it will take several tries after the bang before it will start. I am new to the CNG market so I don't know the names of the various CNG components. Thanks.

The loud bang you heard was probably a back fire. One thing you don't want to do with a gaseous fuel engine is pump the throttle while cranking. When you open the throttle quickly the fuel mixture goes lean because the air flow throught the engine will pick up quicker than the fuel metering system can respond (more air, less fuel = lean mixture). If the lean mixture is ignited by the plug it will burn very slowly and it can be still burning when the next intake stroke starts (durning valve overlap) and the mixture in the intake manifold can be ignited.

Besure to check all the hoses and other connections of the air intake system, including the air cleaner element and housing for leaks, cracks or disconnected hoses - especially if the car is running funny.

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I have the exact same problem in my 2001 cavalier (120,000 miles). I get an occasional backfire while cranking at ignition. If I have to crank for more than three of four seconds, then I might occasionally get a mild backfire. and once it backfires, then it will take even longer to start.
When I posted this as a question on the forum, someone told me never pump the throttle while cranking... but I never did. This got me curious, so I decided to try it, and it actually helps a lot. If I give the car plenty of gas while cranking, I have a lot less backfires.

About a year ago I developed a backfire while accelerating. After replacing plugs and wires, the accelerating backfires stopped, and I was hoping it would fix this too, but it didn't.

My current theory is that as the car sits, some of the CNG in the line between the LPR and the cylinders leaks out, and gets displaced with air (this is somewhat confirmed by the fact that I can occasionally smell a whiff of gas around my air filter a few hours after turning off the car). I think that when this happens, and you try to start the car, you have a very lean mixture (diluted with air) coming into the engine, so you get a backfire. Pumping some gas into the car while cranking may help by guaranteeing that the right concentration of fuel quickly gets to the cylinders. Gasoline doesn't do this, because as a liquid, as long as gasoline is present at the injectors, it is guaranteed to always be the same "concentration" and cannot become diluted with air like the gaseous fuels can. At least that's my theory.

A few weeks ago I had to replace my starter motor, it seems to have helped a little, but if I don't pump the throttle, I still get a backfire on occasion.

KIRK

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Thank you for your help. I looked things over and saw that my air filter cover is cracked. I think that this is from the back fires (they are happening under the hood, not in the rear of the car.) I thought that I saw a cover with one way valves on it to avoid this problem. Am I supposed to have a cover with one way valves on it or just a regular cover?

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The cover with "pop off" valves doesn't work. Backfires happen on these cars and that cover was made to answer field complaints. Chevrolet had a service bulletin about this and owners who complained about the backfires could have this style cover installed at no cost during the warranty period. The backfire is usually more severe then the amount of pressure the valves will pass---like I say, the cover usually breaks to save the valves . The attached photo shows just such a cover with the valves--notice that the valves are in perfect condition! Just go to the junkyard (usually a U pull it junkyard is best) and grab a couple of aircleaner covers for spares. You should also get an entire aircleaner as sometimes the entire housing cracks. Most covers from '95 to '05 work.

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I just bought a used 2001 Cavalier CNG. I also experienced the cracking air filter cover. I saw that this was a common problem. I just replaced my cover. I went to the hardware store (OSH) and purchased longer bolts and washers. I replaced to cover and when I bolted it back in place I put a compression spring between the washers. I might have to play aroung for the right copression spring, but since it goes on a negative pressure location I probably wont have a problem. I will post if this does not help.